On the lifetime of the transients (NP)-(CH ₃) _n (NP=Ag ⁰, Au ⁰, TiO ₂ nanoparticles) formed in the reactions between methyl radicals and nanoparticles suspended in aqueous solutions

Methyl radicals react in fast reactions, with rate constants k>1×10 ⁸-M ^-1-s ^-1, with Au ⁰, Ag ⁰ and TiO ₂ nanoparticles (NPs) dispersed in aqueous solutions to form intermediates, (NP)-(CH ₃) _n, in which the methyl groups are covalently bound to the NPs. These intermediates decompose to form ethane. As n≥2 is required for the formation of C ₂H ₆, the minimal lifetime (τ) of the methyls bound to the NPs, (NP)-CH ₃, can be estimated from the rate of production of the CH ₃ ^. radicals and the NPs concentration. The results obtained in this study, using a very low dose rate γ-source for NP=Ag ⁰, Au ⁰, and TiO ₂ point out that τ of these intermediates is surprisingly long, for example, ≥8 and ≥188 sec for silver and gold, respectively. These data point out that the NP-C bond dissociation energies are ≥70 kJ-mol ^-1. Under low rates of production of CH ₃ ^., that is, when the rate of formation of ethane is very low, other reactions may occur, consequently the mechanism proposed is "broken". This is observed in the present study only for TiO ₂ NPs. These results have to be considered whenever alkyl radicals are formed near surfaces. Furthermore, the results point out that the rate of reaction of methyl radicals with (NP)-(CH ₃) _n depends on n, that is, the number of methyl radicals bound to the NPs affect the properties of the NPs.